Zoom systems for microscopes, particularly for operating microscopes or high performance stereomicroscopes, are known in a number of forms from the prior art. For example, U.S. Pat. No. 6,816,321 discloses an afocal zoom system for high performance stereomicroscopes with which zoom factors z (ratio of maximum to minimum zoom magnification) of more than 15 can be achieved. Besides optional add-on modules, stereomicroscopes of this type comprise a main objective which images the object towards infinity, two parallel refractors downstream thereof, for varying the magnification, and two eyepiece units (binocular tubes) comprising a tube lens, a reversing system and an eyepiece for optical viewing with both eyes. The refractors may be in the form of Galileo refractors for stepwise selection of the magnification or as afocal zoom systems for continuously selecting the magnification. The distance of the refractor axis is termed the stereo base. The numerical aperture of this microscope is half the diameter of the entrance pupil of the telescope or refractor divided by the focal length of the main objective.
German patent DE 102 25 192 B4 relates to an objective for stereomicroscopes of the telescope type and a corresponding stereomicroscope. For details of its construction and mode of operation and the relation between magnification, resolution and vignetting, reference is made expressly to the above patent.
A high powered stereomicroscope with enhanced resolution and at the same time increased depth of focus for the same size of stereomicroscope is known from German patent DE 10 2006 036 300 B4. The embodiments in conjunction with FIGS. 9 and 10 in the specification of that document describe the construction and mode of operation of the afocal zoom systems used for the high powered stereomicroscope described therein. To this extent, reference is made specifically to this specification. Additionally, the afocal zoom systems referred to are also described in U.S. Pat. No. 6,816,321 mentioned previously (corresponding to DE 102 22 041 B4). FIG. 1 of the specification of DE 10 2006 036 300 B4 shows two parallel afocal zoom systems of a stereomicroscope in which an (iris) shutter or (iris) diaphragm with an adjustable diameter is arranged within the zoom system. The diameters are adjusted to be the same for both zoom systems. The iris shutters limit the corresponding diameters of the entrance pupils which are variably large, depending on the zoom setting and choice of shutter, but are the same on both sides.
In another type of zoom system, an (aperture) shutter or (aperture) diaphragm is positioned after the last lens component in the direction of light flow. A corresponding system is schematically shown in FIGS. 1 and 2 of this application. The zoom system here is designated 1 and has four lens components or groups L1, L2, L3 and L4, the two outer lens components L1 and L4 being fixed, while the two inner lens components L2 and L3 are arranged to be axially moveable (along the optical axis). The movement of the lens components L2 and L3 takes place along precisely prescribed pathways and passes from a setting for low magnifications (FIG. 1) to a setting for high magnifications (FIG. 2). The shutter (or diaphragm) arranged after the zoom system 1 in the direction of propagation of the beam is designated B.
FIGS. 1 and 2 of this application show that the shutter (or diaphragm) B is effective at low magnifications while it has little or no effect at high magnifications. The shutter B thus influences the numerical aperture and hence also the depth of focus and brightness only for low to medium magnifications, while the depth of focus at high magnifications, in particular, cannot be increased. Further closing of the shutter B results in strong vignetting for the edge regions of the object field. On the other hand, at low magnification, the depth of focus is great in any case, so that a further increase in the depth of focus using the shutter B is scarcely necessary in practice. Further remarks on this zoom system 1 and on zoom systems in general can be found in the “Selected Papers on Zoom Lenses”, published by Allen Mann, SPIE Milestone Series, Volume MS 85.